This invention pertains to the field of electronic circuits for driving reflective liquid crystal displays (RLCD).
In an RLCD having a matrix of m horizontal rows and n vertical columns, each m-n intersection forms a cell or picture element (pixel). By applying an electric potential difference, such as 7.5 volts (v), across a cell, a phase change occurs in the crystalline structure at the cell site causing the pixel to change the incident light polarization vector orientation, thereby blocking the light from emerging from the electro-optical system. Removing the voltage across the pixel causes the liquid crystal in the pixel structure to return to the initial xe2x80x9cbrightxe2x80x9d state. Variations in the applied voltage level produce a plurality of different gray shades between the light and dark limits.
FIG. 1 illustrates an example block diagram of a conventional column driving arrangement for an RLCD device. A column driver 18 provides a ramp voltage to each of a plurality of column lines 20, progressively applying a voltage corresponding to each gray-scale level. A counter 12 sequentially progresses through each gray-scale value, typically 0-256, although other levels of gray-scale resolution may be provided. A look-up-table LUT 14 maps each gray-scale value to a voltage that corresponds to this value; this mapping is a function of the particular RLCD, and is typically non-linear. The voltage value is converted to an analog voltage level by a digital-to-analog converter (DAC) 16, and this analog voltage provides the input to the driver 18. As discussed further below, the driver 18 is typically a high-current device.
The load that each column line 20 presents to the driver 18 is represented as a capacitance 28, which represents the sum of the capacitances of the individual pixels in the column and the capacitance of the lines to these pixels. Each column line 20 includes a switch 26 that serves as a sample-and-hold gate, wherein the capacitance 28 serves as the xe2x80x9choldxe2x80x9d storage element. Each column switch 26 is controlled by a comparator 24 that compares the current count of the counter 12 to the desired gray-scale level for the column, which is stored in a data memory 22. When the count from the counter 12 reaches the desired gray-scale level for the column, the comparator 24 opens the switch 26, placing the capacitance 28 in the hold-state, holding the current value of the ramp voltage from the driver 18. Not illustrated, a row-controller subsequently applies the voltage on the capacitance 28 to the pixel at the intersection of the column and the selected row.
At the end of each row-cycle, all of the capacitances 28 are discharged and the above process is repeated. Because this discharge must occur quickly (typically within 30 nanoseconds), and must discharge all of the capacitances 28 (typically 5-10 nanofarads), the peak current of the discharge can be as high as a few amperes. In a conventional RLCD, the driver 18 is configured to provide this high-current capacity.
A number of drawbacks can be attributed to the conventional RLCD column driver arrangement of FIG. 1. As noted above, the driver 18 must be configured to accommodate a high discharge current. Additionally, when each switch 26 is opened, a transient is fed back to the driver 18 from the gate of the switch 26. This transient can be substantial, particularly when a large number of switches 26 open simultaneously, such as when a line segment of uniform gray-scale is being displayed. This transient modifies the voltage level from the driver 18, causing it to differ from the voltage provided by the LUT 14 corresponding to the current gray-scale value in the counter 12. Any columns that have not yet entered the hold-state will receive this erroneous voltage, and will display an improper gray-scale level. This transient effect is commonly termed xe2x80x9chorizontal crosstalkxe2x80x9d. Further, the common connection of multiple column lines 20 to the driver 28 provides a substantial xe2x80x9cantennaxe2x80x9d, and is susceptible to noise transients as well.
In this invention, a column driving arrangement for an RLCD device is provided that isolates the source of a ramp voltage corresponding to gray-scale levels from the sample-and-hold gates of the individual columns. Preferably, this isolation is provided by an operational transconductance amplifier (OTA) at each column that provides a controlled current for charging the column capacitance to the appropriate gray-scale voltage level. The capacitor effects an integration of the current, thereby providing a noise-filtering effect. Additionally, a each column capacitance is individually discharged, thereby obviating the need for a common high-current discharge device.